The invention relates to a film which is metallizable, has good machinability and which is heat sealable over a broad temperature range. It is desirable to have a broad sealing temperature range to increase the productivity of packaging machines.
Oriented polypropylene films with thin sealable surface layers of ethylene-propylene copolymers or ethylene-propylenebutene-1 terpolymers have been used in packaging operations. However, these films have not yielded hermetic seals on bags made with the films on packaging machines operated at standard conditions. Hermetic seals are required when leak-free packaging is important. In addition, the inclusion of metallized layers in machineable packaging films is often desirable. Metallized films have proven to be useful in many flexible packaging applications. Such films have often included a polypropylene core with a surface layer to which a metal layer is adhered and another surface layer which is heat sealable.
Important properties of films used on packaging machines are the machinability of the film and the heat sealability of the film. In addition, the formation of hermetic seals may be desired. At the same time when using metallized film, a metal layer must adhere to the underlying film substrate throughout the packaging operation and beyond.
The heat sealability, machinability, lamination bond strength and metal adherence properties of the film each can be improved by various factors which in turn may have detrimental effects on the other properties.
Machinability can be improved by improving the coefficient of friction (COF). In the past, coefficient of friction characteristics of polypropylene and other thermoplastic films have been modified by the inclusion in the polymer core of fatty acid amides which tend to bloom or migrate to the surface in order to reduce the coefficient of friction. In addition, finely divided inorganic anti-block agent has been incorporated into surface or skin layers to reduce the coefficient of friction of the film. The use of these additives is disclosed, for example, in U.S. Pat. No. 4,618,527 which describes a film having fatty acid amide in a polypropylene core and inorganic anti-block in skin layers which can have a thickness up to 10% of the total thickness of the film.
U.S. Pat. Nos. 4,604,322 and 4,692,380 describe a metallizable film having 0.03-0.15% by weight of fatty acid amide in a polypropylene core, a metal adhering surface layer of propylene ethylene copolymer having no slip agent, and a heat sealable polymer coating. However, since fatty acid amide blooms to the surface, when incorporated into the core layer the amide can bloom to a metal-receiving layer and interfere with metal adhesion.
A method was devised for improving metal adhesion in U.S. Pat. No. 4,888,237. This patent describes a metallized polymeric film with an isotactic polypropylene core and a metalreceiving layer which is flame treated to improve metal adhesion. Also present is a heat seal surface layer which can contribute 2.5-9% of the total weight of the film and which can contain 0.4 weight percent anti-block and 900 ppm fatty acid amides. However, the problem still remained to attain a metallizeable film with good metal adherence and good machinability which is capable of forming hermetic seals over broad temperature ranges.
Accordingly, it is an object of the invention to provide a metallizable film with low COF on both film surfaces for good operability on packaging machines, high metal adhesion, high lamination bond strengths, good appearance, which also yields hermetic seals over a broad temperature range on bags made with the film on packaging machines operated at standard conditions.